For mobile radio communications, constant envelope modulation techniques (sometimes referred to as angle modulation) are preferable from the standpoint of existing constraints in mobile transmitter power and because they permit use of high efficiency nonlinear power amplifiers. However, another competing goal of digital mobile communications is spectral efficiency; that is, to support as high a data rate as possible within a confined, narrow bandwidth.
Constant envelope, continuous phase modulation (CPM) techniques have been studied by communications engineers for possible application to narrow band radio communications. One specific method of CPM is quaternary continuous phase frequency shift keying (CPFSK). CPM techniques emphasize both power and bandwidth efficiency within a constant amplitude envelope. Data symbols represented as phase shifts can be expressed as the integral of frequency pulses from the modulator. The shape of the frequency pulse determines the type of CPM modulation. Common pulse shapes include rectangular (MSK) and raised cosine. These frequency pulses last from one to several (L) symbol periods (T) depending upon the degree of intersymbol interference. In general, one of (M) possible symbols is transmitted per symbol period. More detailed descriptions of CPM and in particular quaternary CPFSK may be found in the following articles on this subject: "Advances In Constant Envelope Coded Modulation," by John Anderson and Carl-Eric Sundberg, IEEE Communications Magazine, pp. 36-45, December 1991; "Continuous Phase Modulation," by Carl-Eric Sandberg, IEEE Communications Magazine, pp. 25-38, April 1986; "Continuous Phase Modulation--Part 1: Full Response Signalling," by Tor Aulin and Carl-Eric Sundberg, IEEE Transactional Communications, pp. 196-209, March 1981.
A difficulty with CPM techniques like quaternary CPFSK stems from the nonlinearity of the modulation. Linear modulation techniques such as .pi./4-DQPSK can theoretically be demodulated without intersymbol interference (ISI) using a Nyquist filter. In contrast, nonlinear CPFSK modulated signals cannot be Nyquist filtered and demodulated without intersymbol interference. As a result, CPFSK and other continuous phase modulation techniques have not been used in mobile radio communications because of an inability to deal with this intersymbol interference at the receiver site of each radio. Additionally, CPM signals cannot be filtered to confine their spectrum (which is highly desirable in narrow band mobile radio communications) without further adding to the intersymbol interference problem.
While linear modulation techniques such as .pi./4-DQPSK are desirable from the standpoint of reduced ISI, a serious design drawback with such linear modulation techniques is that they require linear power amplifiers. Linear power amplifiers are inefficient and require considerably more power to transmit at a particular level than nonlinear power amplifiers. This is particularly undesirable in mobile radio communications where battery size and life are limited. Specifically, .pi./4-DQPSK modulated signals possess both amplitude modulated and phase modulated components. Any amplitude modulation in the signal spectrum will be mixed with intermodulation components in a nonlinear power amplifier which results in spreading of the spectrum thereby defeating the intent of narrow band transmissions.
The present invention overcomes these drawbacks by providing an apparatus and method for filtering CPM signals, such as quaternary CPFSK signals, to limit the bandwidth occupied by the modulated signals while simultaneously minimizing the amount of amplitude modulation and intersymbol interference introduced by this filtering process. By designing a modulation filter such as a finite impulse response (FIR) filter to add only a nominal, controlled amount of amplitude modulation, high efficiency, nonlinear power amplifiers may be employed to transmit filtered CPM signals. Although the filtering adds a small amount of amplitude modulation (AM) and intersymbol interference, digital signal processor-based demodulators satisfactorily demodulate this type of signal using sequence estimation algorithms such as the Viterbi algorithm. Accordingly, the present invention filters the CPM signals to narrow the signal spectrum for use in mobile narrowband communication while managing the intersymbol interference resulting from the controlled amount of amplitude modulation added by filtering at a tolerable level. Since only nominal amounts of amplitude modulation are added, high efficiency nonlinear power amplifiers may be employed.
A communications system is provided that includes a plurality of radio transceivers. Each transceiver includes a receiver and a transmitter. Each transmitter includes a constant envelope modulator, such as a quaternary continuous phase frequency shift keying modulator, for modulating a carrier with data and thereby generating a modulated signal having a constant amplitude. A modulator filter limits the bandwidth of the modulated signal and generates a filtered signal which adds a controlled amount of amplitude modulation to the modulated signal. The transmitter uses a nonlinear power amplifier for nonlinearly amplifying the filtered signal. The receiver includes a demodulator having a sequence estimator for estimating transmitted symbol sequences in the presence of intersymbol interference.
The filter may be a digital finite impulse response filter including N filter tap coefficients, each corresponding filter tap coefficient being stored in a corresponding location in a memory. A register stores N samples of the modulated signal. A series of multipliers multiply the N filter tap coefficients with the N samples currently stored in the register to generate N products. An accumulator sums the N products to produce the filtered modulated signal. Alternatively, the digital finite impulse response filter may include memory for storing a table of products at corresponding address locations in the memory, each product being the result of multiplying a particular filter tap coefficient with a particular phase state. The N samples stored in the register are used to address one of the address locations and retrieve the N products for each set of N samples stored in the register.
The amount of amplitude modulation generated by the filter is limited base on an amount of resulting intersymbol interference that can be tolerated by the sequence estimator. In one embodiment, the amount of amplitude modulation generated by the filter is based on the ratio of the maximum amplitude of the modulated signal to the minimum amplitude of the modulated signal.